1. Field of the Invention
This invention relates a light beam scanning system which causes a light beam emitted from a light source and deflected by a deflector to scan a surface.
2. Description of the Related Art
A light beam scanning system is employed in an image read-out system and an image recording system in order to scan a recording medium by a light beam for reading out or recording an image on the recording medium. For example, in a radiation image read out system employing a stimulable phosphor sheet, the stimulable phosphor sheet is scanned by a stimulating light beam and light emitted from the stimulable phosphor sheet upon stimulation thereof is detected. As such a light beam scanning system, there has been known those comprising a light source, a deflector such as a rotary polygonal mirror for deflecting the light beam emitted from the light source, and a scanning/imaging optical system which causes the light beam deflected by the deflector to form an image on the surface of a recording medium and scan the same and is provided with an optical element such as an f.theta. lens for causing the light to scan the surface at a constant speed and correcting curvature of field. An image is recorded on the recording medium or an image is read out from the recording medium by two-dimensionally scanning the surface of the recording medium by the light beam by causing the light beam to scan the surface at a constant speed in one direction (main scanning) while the recording medium is moved in a sub-scanning direction perpendicular to the main scanning direction.
In one type of such a light beam scanning system, the light beam is collimated by a beam shaper and the collimated light beam is caused to scan the surface as disclosed, for instance, in Japanese Unexamined Patent Publication No. 2(1990)-115814, and in another type, the light beam is converged to form a beam spot on the surface and the beam spot is caused to scan the surface in order to suppress curvature of field as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-361218.
However in the light beam scanning systems disclosed in the above identified patent publications where a collimated light beam or a converged light beam is caused to scan the surface, there has been a problem that when the light beam is deflected by an angle larger than 90.degree. to scan a larger area of the surface, f.theta. correction for ensuring that the light beam accurately scans the surface at a constant speed makes it impossible to accurately correct curvature of field. Though both the f.theta. properties and the curvature of field can be accurately corrected by increasing the number of the lens elements in the scanning/imaging optical system, increase in the number of the lens elements increases the size of the system and adds to the cost.